Ask Dave Epstein: What are king tides?

A man stands on a flooded city street. — Water floods Long Wharf during high tide, Friday, Dec. 23, 2022, in Boston.

Dave Epstein is a meterologist who partners with GBH News to provide daily weather updates and weekly higher-level conversations on Morning Edition.

Gal Tziperman Lotan is a digital producer for GBH’s Morning Edition.

December 01, 2023

Jeremy Siegel: You're listening to GBH's Morning Edition. You might have heard the term king tide before or seen videos of the Seaport and other parts of the coast flooding when the king tide comes in. But what exactly is it?

Paris Alston: To answer that question, we are joined now by GBH meteorologist Dave Epstein. Good morning, Dave. Rolling in like a king tide yourself. How are you?

Dave Epstein: I'm good. How are you? Yeah, just outside for a little walk with the dog. And a little chilly this morning, but fairly typical for the final day of November.

Siegel: So, Dave, what is a king tide?

Epstein: Yeah, so it's basically a nonscientific term people use to describe the high tide that occurs during a new or a full moon. So the position of the moon in relation to where we are changes every 28 days. It's changing actually every day. But we get, you know, you get a full moon roughly every 28 days or so. And when you get the full or the new moon, there's more pull on the earth from the moon. So you get bigger tides. So if you were to look at a chart of tides over the course of every single month, you'd see a high tide that's a little lower than about average, then a little higher than average, and then back to a little lower and then the ultimate low tide again. And sort of it's this sort of wavy cycle that goes through the course of the year. And then within that, during the year, there are some months where the tide is even higher. So those peaks also change. It's not exactly the same every month.

Siegel: So it's when the high tide gets so darn high that we just have to slap the word king on top of it.

Epstein: Yeah, exactly. It's just a little bit of a higher tide than average. It's like the supermoon, right? It's like, oh, there's always a full moon every month. But some months during the course of some years, the moon's just a little bit bigger. So what do we do? We have to name it and we give it the supermoon. It's the same thing.

Alston: Oh, man. It's always just so fascinating to think about how the moon controls our tides down here on Earth. And Dave, this obviously happens time and time again. But how has it changed over time?

Epstein: Yeah, that's a great question. So the king tides have been happening, you know, since the planet's really been around. And the height of the tide is not necessarily determined by the moon alone. So it's topography as well. Our tides up in the Bay of Fundy, as you head north up into Maine, even Boston Harbor gets a 11, 12-foot high tides. Big difference, right? Zero to 12 feet. That's a huge change over the course of those six hours where if you went down to places like Florida, the Gulf Coast, the tides are 1 or 2 feet, much, much less. Over time, because water expands as it gets warmer, the thermal expansion of water, our tides are getting higher. So we're gaining about .86 feet every century, so just under a foot every 100 years. And that is increasing in terms of rate. Now, that doesn't sound like a lot, but a foot of water, right? Between 1923 and 2023 at a high tide like a king tide — and then you add in, say, a nor'easter — that's a big deal. So what planners look for, you know, when they're building buildings, when we're managing the Seaport and all of that, is the fact that, okay, what is it going to be like in 2123, another foot, at least higher? Now, you added in a nor'easter, now you add in a king tide, now you have problems in Seaport, Back Bay, places like that. And that's what the concern is and the connection between a changing climate and what's always happening, which are bigger tides.

Siegel: We're talking with meteorologist Dave Epstein about king tides and flooding along the coast. Dave, this is something I've seen firsthand right next to my neighborhood in the North End during these king tides. I've seeing waters not just like right up where people would be walking around the harbor, but flooding like right up against restaurant or hotel spaces. I mean, how are you thinking about where we've chosen to live or where we're choosing to live going forward? Like, is there anything that we can figure out here?

Epstein: Yeah. I mean, I think that, you know, building codes and things like that, city planners are definitely going to be incorporating this into future buildings. So if you look down at the Seaport District, for example, when they redid one of the buildings down there, I'm drawing a blank — the World Trade Center, when they redid that, right, they were planning for the fact that the tides are going to be higher. And that's the case throughout, you know, all these places around the coast because tides are going to be changing and are going to be higher over time.

Alston: Well, that is GBH meteorologist Dave Epstein. And a reminder that if you ever have a question for Dave, you can text that to us at 617-300-2008 or email thewakeup@WGBH@.org. Dave, thank you so very much.

Epstein: You're welcome. Good to be here.

Siegel: You're listening to GBH's Morning Edition.

Meteorologist Dave Epstein is our go-to person for pressing weather questions on everything from winter blizzards to summer droughts. He’s also a horticulturist, meaning he’s an expert in anything that grows leaves and flowers. GBH's Morning Edition asked our audience for weather and gardening questions, and Epstein graciously answered them on the air. This transcript has been edited for clarity.

Have a gardening or weather question for meteorologist Dave Epstein? Tweet him @GrowingWisdom, email us at thewakeup@wgbh.org, or text 617-300-2008.

What are king tides? Are they becoming more common?

You may have heard the term king tide, often in reference to coastal flooding, or seen waves lapping onto sidewalks along the Boston coastline.

But what is a king tide, exactly?

“It's basically a nonscientific term people use to describe the high tide that occurs during a new or a full moon,” Epstein said.

Higher tides during full or new moons have always existed, Epstein said.

“When you get the full or the new moon, there's more pull on the earth from the moon, so you get bigger tides,” he said. “If you were to look at a chart of tides over the course of every single month, you'd see a high tide that's a little lower than about average, then a little higher than average, and then back to a little lower and then the ultimate low tide again.”

Many other factors can contribute to higher-than-usual tides. Big storms, like nor'easters, can cause storm surges. Topography plays a role too — tides along the Boston Harbor can be about 10 feet, according to National Oceanic and Atmospheric Administration data.

“Big difference, right?” Epstein said. “That's a huge change over the course of those six hours where if you went down to places like Florida, the Gulf Coast, the tides are 1 or 2 feet — much, much less.”

While king tides themselves are not a result of climate change, warmer waters and sea level rise can lead to higher tides than we’d expect without it.

“Over time, because water expands as it gets warmer — the thermal expansion of water — our tides are getting higher,” he said. “We're gaining about .86 feet every century, so just under a foot every 100 years. And that is increasing in terms of rate.”

That might not sound like a lot, he said, but it can make a real difference during high tide. And cities built centuries ago were not always designed with those high tides in mind.

Going forward, he said, urban planners and developers should be consider higher tides when designing along the coast.

“City planners are definitely going to be incorporating this into future buildings,” he said. “Okay, what is it going to be like in [the year] 2123, another foot, at least higher? Now, you added in a nor'easter, now you add in a king tide, now you have problems in Seaport, Back Bay, places like that. And that's what the concern is and the connection between a changing climate and what's always happening, which are bigger tides.”